The hard drive heats up pretty good - could overheating be a factor?
After all, it sits in the enclosure under the seat and there is only one fan which doesn't cool the hard drive, only its power supply unit.

I assume you are talking about a desktop hard drive? If so I doubt that heat is an issue here, more the additional shocks the hard drive gets from being in a car - its a pretty unfriendly enviroment in there and they aren't really meant to be moved around a whole lot.

You could try mounting it on rubber or something but I don't see it as any kind of solution. Laptop hard drive's are designed to take some impact so are a far better bet in a car IMHO, although you do have to pay for this additional protection.

Hard drives (and other computer parts), like everything, can sometimes come bad from the factory. Maybe the little movement it went through was enough to just prompt the errors to appear more than before.

I've been using a 3.5" Seagate Barracuda drive for about 6 months now, with no problems at all.

I did however have a couple of problems with a Maxtor drive, one of the newer FBD (Fluid Bearing Drives) ones. It wouldn't work properly in the car during the winter. Moved it inside, and put the Seagate in the car, and have had no problems with either since.

It could be DOA. But I didn't experience any skipping during first few days of operation.
I will do a throughout surface check this time and then we'll see if this happens again.
After doing the incomplete surface check, skipping almost stopped.
It's a good thing, because now I know the source of the problem.
If you're saying that heat isn't an issue, I will most likely wrap the enclosure in plastic "bubbles" - they are great shock absorbands and I have a sheet of just the right size.

I've never had a problem myself (in over a year) - and that's including accidentally driving over a brick while the HDD was spinning. That's in an MR2 which had pretty hard suspension and now a Ford Focus with sports suspension, driving daily on harsh British country roads with no concern for the HDD!

You can usually find the maximum operating shock the drive will support on the manufacturer's web site - yes 2.5's generally are more tolerant to shock.

Mine isn't mounted in anything special, it's screwed as normal to the case chassis. The only different is the drive is mounted vertically, so the spindle is facing the same direction as the wheels.

Other people have also added rubberised mounts to absorb some of the shock but even that could be overkill as long as it is mounted in a way to minimise the chance of a head crash.

I wouldn't go as far as to say heat isn't an issue for a HD, but in this instance I think vibration is more likely to be the culprit - although as Confused points out a bad drive is a distinct possibility.

If the drive is new I would definitely return it, I wouldn't advise covering it in bubble wrap though as this could make heat an issue. Modern HD's can get pretty warm under load, instead try mounting it as advised by stevieg - UK roads can be pretty poor and if he's had good results with it mounted vertically with the spindle facing the same way as the wheels then it should work for you too. Good luck!

Thanks, that should work
I tried calling support, they told me that if the drive works they can't exchange it.
They suggested running full surface scan in DOS, and that's what I'm doing now.
It's a lot faster than Windows ScanDisk and I can see visual progress.
Here's a picture of my HD's surface after 1/3 of the bad clusters were identified.
I will put another picture once I make a full scan to give you an idea of the damage:

I've never had a problem myself (in over a year) - and that's including accidentally driving over a brick while the HDD was spinning. That's in an MR2 which had pretty hard suspension and now a Ford Focus with sports suspension, driving daily on harsh British country roads with no concern for the HDD!

You can usually find the maximum operating shock the drive will support on the manufacturer's web site - yes 2.5's generally are more tolerant to shock.

Mine isn't mounted in anything special, it's screwed as normal to the case chassis. The only different is the drive is mounted vertically, so the spindle is facing the same direction as the wheels.

Other people have also added rubberised mounts to absorb some of the shock but even that could be overkill as long as it is mounted in a way to minimise the chance of a head crash.

#1 in theory is the worst. #2 is how mine is mounted and #3 isn't much different to #2 - as we're trying to make sure the drive heads aren't jilted in the direction of the plater surfaces.

Regarding the supplier not replacing the disk - if you drove only a few miles I doubt that damaged it. I have on occasion driven around with the HDD in position #1 and had no damage - though I know if I did hit a big pothole I'd prolly have a head crash.
There will probably be a certain bad-block threshold that would allow them to send back as DOA - if it can't even play MP3s properly due to errors then it's not working - it's not fit for purpose and they don't have a leg to stand on.

Here's an idea - if they reckon it is OK then surely they won't mind swapping it and re-selling it.

By the way, i'm using a 5400 Maxtor 30GB - Maybe 5400 does make a difference?